In general, prior known pneumatic atomizer and nebulizer devices are based upon a principle in accordance with which a propellant gas is forced through a narrow orifice into contact with a thin film or stream of liquid which is fed to the periphery or outlet of the orifice. At this location the thin film or stream of liquid is entrained in the propellant gas flowing out of the orifice and broken into droplets, which are carried away by the flowing gas.
Such known pneumatic nebulizers and atomizers have several disadvantages. Most such nebulizers are not effective in emitting a fog of liquid particles which is both dense and composed of fine liquid particles when operated with the propellant gas at pressures less than about 5 p.s.i. If the propellant gas is at a pressure less than about 5 p.s.i., either the fog emitted by the pneumatic atomizer will be thin, or the liquid particles within the fog will be large, depending on the design of the pneumatic atomizer and on the amount of liquid supplied to the pneumatic atomizer. If the propellant gas pressure is less than 5 p.s.i., and the amount of liquid supplied to the pneumatic atomizer is not sharply reduced, the liquid particles in the emitted fog will be unacceptably large, with resulting fall-out of liquid from the emitted fog.
The foregoing difficulties are partly ameliorated in some pneumatic atomizers designed for low pressure propellant gas by placing an impactor, shroud or other barrier in the path of the emitted fog to separate out those liquid particles having particle sizes above about 50 microns. Such known pneumatic nebulizers cannot directly produce a fog having dispersed liquid particles have a maximum diameter of 20 microns or less.
If the fog contains liquid particles larger than about 20 microns in diameter, the larger liquid particles in the fog will strike the impactor and wet its surface, whereas the smaller particles in the fog will be carried around the impactor by the propellant gas and will not wet the impactor's surface. The difficulty with placing an impactor or other barrier in the path of the emitted fog to capture larger particles in the emitted fog is that a means must be provided to collect the liquid that comes into contact with the impactor or barrier, and a means must be provided to recirculate the collected liquid or otherwise dispose of the collected liquid.
The relevant patents are the Metcalf Patent No. 1,436,351 entitled "Fuel Nozzle," which issued Nov. 21, 1922; the Erb and Resch Patent No. 3,993,246 entitled "Nebulizer and Method," which issued Nov. 23, 1976; and the Erb and Resch Patent No. 4,018,387 issuing Apr. 19, 1977 and entitled "Nebulizer," which is a division of the immediately preceding patent. Other relevant patents are the Erb and Resch Patent No. 4,161,281 entitled "Pneumatic Nebulizer and Method," issued Jul. 17, 1979; the Erb and Resch Patent No 4,161,482 entitled "Microcapillary Nebulizer and Method," also issued on Jul. 17, 1979; and the Erb and Resch Patent No. 4,261,511, entitled "Nebulizer and Method," which issued Apr. 14, 1981.
The devices covered by the foregoing patents may be regarded as comprising the following elements:
1) A surface on which the liquid to be atomized is spread, resulting in a film of the liquid on the surface;
2)One or more orifices that pass through the filming surface; and
3) A means for supplying gas to the under (back) side of the filming surface, such gas being at a greater pressure upstream of the filming surface than the ambient gas above the filming surface.
It is important to note in this context that the pressure of the gas upstream of the filming surface may be at atmospheric pressure if the ambient pressure over the filming surface is at a vacuum, as is the case in an internal combustion engine intake manifold. The consequential point is that there be a pressure drop between a point upstream of the filming surface and the front side of the filming surface to cause the gas to flow from such point, through the orifices in the filming surface, to the front side of the filming surface. This drop in pressure is called the pressure head.
In operation, gas flowing through the orifices in the filming surface entrains liquid drawn from the liquid film on the filming surface, which entrained liquid is drawn into ribbons, which ribbons break into shreds, which shreds collapse into droplets. The droplets are then carried off by the flowing gas.
To generate fine liquid particles, (i) the liquid film must be as thin as possible where it meets the flowing gas; (ii) the conditions where the liquid film and flowing gas meet should be such as to encourage the liquid in the liquid film being entrained in the flowing gas as thin ribbons of liquid; and (iii) the flowing gas should be moving at the point where it encounters the liquid with the highest velocity obtainable with the available pressure head.
The prior art, such as the patent to Metcalfe, U.S. Pat. No. 1,436,351 and the Erb and Resch U.S. Pat. Nos. 4,161,281 and 4,161,282 teach various means and devices for making a thin liquid film on a filming surface that has one or more orifices through the filming surface. The prior art does not teach designing the atomizer to enhance the entrainment of the liquid into the flowing gas as thin ribbons of liquid, nor does the prior art teach designing the atomizer to maximize the velocity of the gas flow at the point where the flowing gas encounters the liquid. Significantly, the prior art does not teach a nozzle defined by a smooth converging surface or duct which guides the flowing gas from a large cross-sectional area conduit to the underside of the filming surface, the outlet of the nozzle almost matching the shape and cross-sectional area of the orifice through the filming surface.
Most importantly, the prior art does not teach the utilization of a sharp edge orifice in the filming surface through which the flowing gas passes, which orifice is slightly smaller in cross-sectional area than the outlet of the nozzle, with a short gap or separation being created between the sharp edge of the orifice and the location where the flow of gas through the orifice comes into contact with the liquid entrained from the filming surface.
An examination of the prior art discloses the pressurized gas used to operate the pneumatic atomizer is supplied by means of a conduit that directs the pressured gas to a chamber within the pneumatic atomizer. This chamber is hereinafter called the "the gas chamber." The gas chamber has one or more orifices passing through a wall of the gas chamber to the exterior of the atomizer. Such orifices are hereinafter called "the gas orifice." The exterior surface of such wall serves as a filming surface on which is located the liquid to be atomized. The liquid to be atomized is directed onto the filming surface as a thin film, which film extends around the periphery of the gas orifice.
The inner wall of the gas chamber near and about the inner edge of each gas orifice is approximately perpendicular to the centerline of the gas orifice. Stated in other words, the width of the gas chamber measured at the inner edge of the gas orifice is substantially greater than the width of the gas orifice. This means the pressurized gas passes from a space of relatively large width to a space of relatively small width as the pressurized gas passes from the gas chamber into the gas orifice. It also means the transition occurs suddenly. The sudden transition is due to the approximately right angle relationship between the sidewall of the gas orifice and the inner adjoining wall of the gas chamber.
The approximately right angle relationship of the sidewall of the gas orifice and the adjoining inner wall of the gas chamber is hereinafter called a "sharp edge." The gas orifice's sharp inner edge and the laws of fluid dynamics applicable to the flow of a pressurized gas flowing from a large container through a small sharp edged orifice in the wall of the container results in the gas exiting the gas orifice with a velocity that is not constant across the width of the gas orifice.
The gas flowing through the center of the gas orifice will have the fastest velocity, whereas the gas flowing through the gas orifice near the periphery of the gas orifice will have the slowest velocity. The difference in velocity can be substantial.
With continuing reference to the prior pneumatic atomizer art, the fact that the gas flowing near the edge of a gas orifice has a much slower velocity than the gas flowing near the center of the gas orifice, has a very detrimental effect on the pneumatic atomizer's ability to atomize the liquid film on the filming surface.
It is important to realize that it is the gas near the periphery of the gas orifice that encounters the liquid film about the periphery of the outlet of the gas orifice; entrains the liquid film; draws the liquid film into ribbons that break into droplets; and then carries the droplets off. It clearly is not the gas flowing through the center of the gas orifice that entrains the liquid.
It is also a fact that the gas near the periphery of the gas orifice is not able to atomize into fine particles as much liquid as the gas could if the gas near the periphery of the gas orifice were flowing at the higher velocity of the gas to be found in the center of the gas orifice.
It is therefore a very important object of this invention to provide a highly advantageous pneumatic atomizer configured to cause the velocity of the gas flowing near the periphery of the gas orifice to be almost the same as the velocity of the gas near the center of the gas orifice.